Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation; full citations for these documents may be found at the end of the specification. The disclosure of the publications, patents, and published patent specifications referred in this application are hereby incorporated by reference into the present disclosure.
Glass is an inorganic product of fusion that has cooled to a rigid condition without crystallizing (ASTM C-162). The most common glasses are silicate glasses. The basic structural unit of silicate glasses is the silicon-oxygen tetrahedron in which a silicon atom is tetrahedrally coordinated to four surrounding oxygen atoms. Similar to the crystalline silicates, the SiO4 tetrahedra in the silicate glasses are found in a variety of configurations depending on the oxygen-to-silicon ratio in the glass compositions.
Some glasses are naturally occurring, such as perlite, pumice, obsidian, pitchstone, and volcanic ash. Others, such as soda-lime glasses, are produced synthetically. For example, soda-lime glass may be made by melting batches of raw materials containing the oxides of silicon (i.e., SiO2), aluminum (i.e., Al2O3), calcium (i.e., CaO), sodium (i.e., Na2O), and sometimes potassium (i.e., K2O), or lithium (i.e., Li2O) together in a furnace, and then allowing the melt to cool so as to produce the amorphous product. Glasses may be made in a wide variety of shapes, including sheets or plates, cast shapes, or fibers. Methods of manufacturing the principal families of glasses have been reported (Scholes, 1974). Mineral wools, rock wools, and silicate cottons are generic names for manufactured fibers in which the fiber-forming substances may be slag, certain rocks, or glass (Kujawa, 1983).
Foam glasses are a special class of lightweight glass materials having numerous completely sealed small cells. The process of making foam glasses has been developed over many years and the most common technique of making foam glasses consists of following steps: 1) melting of glass raw material at high temperature to form a base glass, 2) grinding the base glass with additional foaming agents, 3) foaming of the ground glass powder at high temperature. The base glass composition is similar to the regular window glass which typically contains 70-73% SiO2, 1-3% Al2O3, 0.1-0.5% Fe2O3, 13-15% Na2O, 0-2% K2O, 5-7% CaO and 3-5% MgO (by weight). The foaming agents are normally carbon black and alkali carbonates.
Other techniques have also been used to make foam glasses. For example, by leaching out the borate phase from a borosilicate glass, a silica-rich phase with very fine pores (10 to 25 Å) is obtained (Elmer, 1971). The moisture trapped in the fine pores by leach solution causes the fine pores to expand after heating the leached glass at 1300-1425° C. by flash-firing. The foaming and sintering of the porous glass particles occurs simultaneously. Due to the residual moisture left in the glass body which increases thermal conductivity, the final foam glass product thus prepared has less desirable insulating property. Alternately, foam glasses can also be made by blowing air or other gases into molten glass and allowing the molten glass to cool and entrap the bubbles or cells in the solidified glass.
The starting materials for commercially manufactured foam glasses are typically virgin glasses. To reduce the cost, various low cost amorphous materials have been used. Recycled mixed color cullet glass has been used to make foam glasses (Solomon, 1996). The waste glass is washed and passed through a magnetic separation step before being passed to a hammer mill or similar type crusher where the separated glass is crushed to a desired particle size. The crushed glass particles and a foaming agent such as CaCO3 or CaSO4 are sized and mixed. The mixtures are placed in molds and passed through a furnace where the mixture is heated to and maintained at a foaming temperature and then cooled or annealed to produce foamed glass blocks.
Minerals have also been used as the starting materials for the foam glass products. For example, diatomaceous earth (natural, calcined and flux calcined), fly ash or their mixture were used to make foam glass (Hojaji, 1984). Skin-surfaced foam glass heat-insulating tiles have been made from vitrifiable minerals such as liparite, rhyolite, pearlite, obsidian and volcanic ash (Fukumoto, 1991).
Conventional perlite products are normally produced by binding expanded perlite particles or a mixture of expanded perlite particles, gypsum, cement and reinforcing fibers with organic or inorganic binders (Alhamad, 1990, Shepherd, 1993, Sun, 2000). The conventional perlite products possess less compressive strength and dimensional stability than is desirable. The conventional expanded perlite products also disadvantageously absorb moisture and do not have optimal water-resistance properties.